The present invention relates to automatic gain control circuits, and more particularly to an automatic gain control circuit for phase sensitive detectors, such as interferometers for electrooptic, microwave or radio frequency (rf) systems as well as coherent detection systems, where the output is desired to be independent of fluctuations in the amplitude of an input signal.
An optical interferometer for an electrooptic system receives a beam of light and separates it with a beam or power splitter into two beams that travel separate optical paths, which beams have a phase or frequency difference between them determined by pressure, light, electricity, or the like. One of the paths is considered to be a "reference" path, and the other a "signal" path, although this distinction is arbitrary since it is the difference between the paths that is important. The beams are subsequently reunited by a power coupler and detected by appropriate photodetectors to produce a differential pair of output currents that represent the phase difference between the two optical paths. Alternatively for a coherent detection system one path includes a local oscillator signal as a reference and the other a phase or frequency modulated signal. The above description applies to microwave and rf systems as well as optical systems. To determine the phase difference between the modulated path and the reference path, either only one of the output currents have been used, or the difference between the output currents have been used. Alternatively the output current waveforms have been digitized and the results processed by software.
It has been suggested by Kurt J. Weingarten in a December 1987 dissertation entitled "Gallium Arsenide Integrated Circuit Testing Using Electrooptic Sampling" that first order intensity noise can be suppressed by normalizing the measured signal to the intensity fluctuations, i.e., using ratio detection. As shown in the block diagram of FIG. 2.12 a differencer & summer produces the sum and difference voltages from the input voltages, and then a divider obtains the ratio between the difference and sum voltages. Barrie Gilbert in the IEEE Journal of Solid-State Circuits, Vol. SC-3, No. 4, December 1968, and in U.S. Pat. No. 3,689,752, describes a wide band amplifier suitable for integrated circuits that has found common use as a multiplier. At page 358, FIG. 7 a multiple input version of the Gilbert circuit is shown where the inputs are known to be in a certain ratio with varying absolute values and it is desired to standardize the signals to a known amplitude. Further FIG. 8 shows the Gilbert circuit with a feedback connection to produce an output equal to the products or quotients of several inputs.
What is desired is a simple automatic gain control circuit for a phase sensitive detector that may be implemented in an integrated circuit and that reduces the unwanted sensitivity to amplitude variations in the input signals by using a ratio detector without the need for feedback.